1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer employing an electrophotographic method.
2. Related Background Art
FIG. 9 is a schematic sectional view showing an example of a conventional image forming apparatus.
Such an image forming apparatus has a photosensitive drum 1 being a latent image bearing member, a charging roller 2 being a charging member employing a so-called contact charging method, a developing means 3, a cleaning device 5 and a fixing means 9.
In such an image forming apparatus, the charging roller 2 to which a voltage (e.g., a direct current voltage, a superimposed voltage of a direct current voltage and an alternate current voltage, or the like in the order of 1 to 2 kV) is applied from a power source 21 is caused to contact the surface of the photosensitive drum 1 as a charged member, whereby the surface of the photosensitive drum 1 is charged to have a predetermined potential (Vd).
Then, in accordance with the rotation (in an arrow a direction in the figure) of the photosensitive drum 1, a laser beam L1 emitted from an exposing means 8a is irradiated on the photosensitive drum 1 that is charged as described above via an exposing window 6a, whereby an electrostatic latent image is formed on the photosensitive drum 1.
In addition, a non-magnetic developing sleeve 3a being a developer carrying member, which is arranged in an opening part provided on the photosensitive drum 1 side of the developing means 3 and fixedly encloses a magnet roll 3c having a plurality of N and S poles, takes toner form an S2 pole being a toner capturing pole to carry it and rotates in an arrow b direction in the figure. Toner on the developing sleeve 3a is regulated by a toner layer thickness regulating member 3b and given predetermined triboelectricity to be coated in a predetermined amount. The developing means 3 is provided with rollers 209 at both ends in an axial direction of the developing sleeve 3a as shown in FIG. 12. As the rollers 209 contact the photosensitive drum 1, a predetermined gap is formed between the developing sleeve 3a and the photosensitive drum 1. When a voltage (e.g., a superimposed voltage of a direct current voltage and an alternating current voltage) from the power source 31 is applied to the developing sleeve 3a, toner performs a so-called jumping phenomenon and reversely develops an electrostatic latent image on the photosensitive drum 1 to visualize it as a toner image.
On the other hand, a transferring material P being a recording medium such as paper is contained in a sheet feeding cassette 117, fed by a sheet feeding roller 118 and synchronized with the toner image on the photosensitive drum 1 by a registration roller (not shown) to be conveyed onto a transferring roller 4.
Toner images on the photosensitive drum 1 are transferred one after another onto the transferring material P that is synchronized with the rotation of the transferring roller 4 provided in an image forming apparatus main body and conveyed.
The transferring material P on which the above-mentioned toner image is transferred is separated from the surface of the photosensitive drum 1, conveyed to a fixing means 9 provided in the image forming apparatus main body, fixed the above-mentioned toner image thereon, and discharged to the outside of the image forming apparatus main body.
On the other hand, transfer residual toner that has not been transferred and remains on the photosensitive drum 1 is removed by a cleaning blade 5a inside the cleaning device 5. The surface of the photosensitive drum 1 from which the transfer residual toner is removed is charged by the charging roller 2 again and served to the above-mentioned process.
As described above, in an image forming apparatus of a transferring method, transfer residual toner remaining on a photosensitive drum after transfer is removed from the surface of the photosensitive drum by a cleaner (cleaning device) to be waste toner. It is preferable that this waste toner is never produced from the viewpoint of environmental protection.
Thus, in a conventional image forming apparatus, an image forming apparatus of a toner recycling process has also been developed which has a configuration for removing transfer residual toner on a photosensitive drum after transfer with the “cleaning simultaneous with developing” by a developing apparatus without a cleaner and collecting the transfer residual toner in the developing apparatus to recycle it.
The cleaning simultaneous with developing is a method of forming a latent image by charging and exposing a photosensitive drum at the time of development in the next and subsequent steps, that is, continuously, and collecting toner remaining on a photosensitive drum after transfer by a fog eliminating bias (a fog eliminating bias Vback that is a potential difference between a direct current voltage applied to a developing apparatus and a surface potential of the photosensitive drum) when the latent image is developed. According to this method, since the residual toner is collected in the developing apparatus and recycled in the next and subsequent steps, waste toner can be eliminated and labor required for maintenance can be reduced. In addition, since the image forming apparatus does not use a cleaner, the image forming apparatus has a significant advantage in terms of space and can be substantially miniaturized.
In addition, conventionally, there is known two types of charging mechanisms of (1) a discharge charging mechanism and (2) a direct injection charging mechanism as a charging mechanism (a mechanism of charging, a charging theory) of contact charging. Each of the charging mechanisms has advantages and disadvantages compared with the other.
(1) Discharge Charging Mechanism
This is a mechanism of charging a surface of a body to be charged by a discharge phenomenon that occurs at a very small gap between the body to be charged and a charging member for contacting the body to be charged to charge the body to be charged (hereinafter referred to as a contact charging member). Since the discharge charging mechanism has constant discharge threshold value in the contact charging member and the body to be charged, it is necessary to apply a voltage larger than a charging potential to the contact charging member. In addition, although a generated amount of ozone is markedly less than that of a corona charger, it is principally inevitable that a discharge products is generated. Moreover, there may also be a problem in that substances in a generated discharge product and a transferring material act each other to hinder formation of a latent image, that is a problem called “smeared image”.
(2) Direct Injection Charging Mechanism
This is a system in which a charge is directly injected in a body to be charged from a contact charging member, whereby the surface of the body to be charged is charged. This is also referred to as a direct charging, an injection charging or a charge injection charging. More specifically, this mechanism is for causing a contact charging member of a medium resistance to contact a surface of a body to be charged and directly injecting a charge in the surface of the body to be charged not via a discharge phenomenon, that is without basically using discharge. Thus, even if a voltage applied to the contact charging member is less than a discharge threshold value, the body to be charged can be charged to a potential equivalent to the applied voltage.
In this way, with the direct injection charging mechanism, there is a significant advantage in that an adverse influence due to a discharge product does not occur because it does not involve generation of an ion. Thus, various patent applications have been filed in the past for the direct injection charging mechanism. For example, in Japanese Patent Application Laid-open No. 10-307454, it is proposed to cause electrically conductive particles to intervene between a charging member and a photosensitive drum. In this application, as shown in FIG. 10, electrically conductive particles supplying means 42 is provided on the upstream side of a charging roller 2, and the electrically conductive particles are supplied between the charging roller 2 and a photosensitive drum 1, whereby the direct discharging mechanism is realized.
In addition, an example of using the direct injection charging mechanism to realize a cleanerless system is disclosed in Japanese Patent Application Laid-open No. 10-307455. According to the application, this system is realized by actions described below.
Electrically conductive fine particulate matters having conductivity that are contained in a developer of developing means are transferred to the side of a latent image bearing member in an appropriate amount together with toner at the time of toner development of an electrostatic latent image on the side of the latent image bearing member by the developing means.
A toner image on the latent image bearing member is pulled by an influence of a transfer bias and actively transfers to the side of a transferring material in a transferring portion of transferring means. However, the electrically conductive fine particulate matters on the latent image bearing member do not actively transfer to the side of the transferring material because they are electrically conductive, and are substantially deposited and held on the latent image bearing member to remain there.
In addition, since the image forming apparatus of the toner recycle process does not use a cleaner, transfer residual toner remaining on the circumference surface of the latent image bearing member after transfer and the above-mentioned residual electrically conductive fine particulate matters are carried to a contacting part of the latent image bearing member and the contact charging member as they are with the movement of the circumference surface of the latent image bearing member and deposited and mixed in the contact charging member.
Therefore, contact discharging of the latent image bearing member is performed in the state in which the electrically conductive fine particulate matters exist in the contacting part of the latent image bearing member and the contact charging member.
Due to the existence of the electrically conductive fine particulate matters, even if toner is deposited and mixed in the contact charging member, precise contacting nature of the contact charging member with the latent image bearing member and a contacting resistance can be maintained. Thus, it is possible to configure the image forming apparatus using a simple member such as a charging roller or a fur brush as the contact charging member. Moreover, direct injection charging of the latent image bearing member by the contact charging member is possible regardless of contamination of the contact charging member due to transfer residual toner.
That is, the contact charging member closely contacts the latent image bearing member via the electrically conductive fine particulate matters, and the electrically conductive fine particulate matters existing in the contacting part of the contact charging member and the latent image bearing member are rubbed between the contact charging member and the surface of the latent image bearing member without any space. Thus, charging of the latent image bearing member by the contact charging member is predominated by the direct injection charging that is stable and safe without using discharge phenomenon due to the existence of the electrically conductive fine particulate matters, and a high charging efficiency can be attained that has not been attained in the conventional roller charging or the like, whereby a potential substantially equivalent to a voltage applied to the contact charging member can be given to the latent image bearing member.
In addition, the transfer residual toner deposited and mixed in the contact charging member is gradually discharged onto the latent image bearing member from the contact charging member to reach the developing portion with the movement of the circumference surface of the latent image bearing member and cleaned simultaneously with being developed (collected) in the developing means (the toner recycle process).
Moreover, even if the electrically conductive fine particulate matters fall from the contact charging member, the image forming apparatus is activated, whereby the electrically conductive fine particulate matters contained in the developer of the developing means transfer to the circumference surface of the latent image bearing member in the developing portion and are carried to the charging portion through the transferring portion by the movement of the circumference surface of the image bearing member to be continuously supplied to the contact charging member sequentially. Thus, the favorable chargeability due to the existence of the electrically conductive fine particulate matters is steadily maintained.
Therefore, in the image forming apparatuses of the contact charging method, the transferring method and the toner recycle process, a simple member such as a charging roller and a fur brush is used as a contact charging member, whereby ozoneless direct injection charging can be steadily maintained for a long period under a low applied voltage regardless of contamination of the contact charging member by transfer residual toner.
The above-mentioned proposals realize a uniform electricity charging property of a surface of a latent image bearing member by the above-mentioned actions and are effective with respect to environmental protection and miniaturization of an apparatus in terms of realizing toner recycling, simply configured and low cost image forming apparatuses without faults due to an ozone product, a charging defect or the like.
However, in the conventional image forming apparatus, if the configuration disclosed in Japanese Patent Application Laid-open No. 10-307454 is used, it is likely that a predetermined Vd is not obtained on a photosensitive drum and a charging defect is caused.
According to studies of researchers or inventors, occurrence of a charging defect was caused by an insufficient absolute amount of electrically conductive fine particulate matters on a charging roller, and the image forming apparatus employed a mechanism for supplementing the fall of the electrically conductive fine particulate matters from a contact charging member by the supply of new electrically conductive fine particulate matters by developing apparatus. Thus, a supply amount of the electrically conductive fine particulate matters by the developing means was not enough in some cases. As a result, an absolute amount of the electrically conductive fine particulate matters on the charging roller used to be insufficient. The researchers further investigated factors for this and found it was a major factor that an amount of electrically conductive fine particulate matters flying on the photosensitive drum 1 from the developing sleeve 3a was not enough. This will be described with reference to FIG. 11.
FIG. 11 is an enlarged model view of a cross section of a gap part between a developing apparatus in which the above-mentioned charging defect has occurred and a photosensitive drum (hereinafter referred to as an S-D gap part).
In the configuration shown in FIG. 11, toner is charged to have a negative polarity and reversely developed in a latent image portion. Electrically conductive fine particulate matters as additives are charged to have a positive polarity, and some of them are served for development in a latent image portion on a photosensitive drum while it sticks to the toner, and some are removed from the toner and flown to a non-image portion on the photosensitive drum to deposit there. That is, as shown in FIG. 2, a developing bias that is a direct current (indicated by Vdc in FIG. 2) superimposed with an alternating current voltage is applied to a developing sleeve. The electrically conductive fine particulate matters sticking to the toner is flown onto a portion with a latent image potential V1 on the photosensitive drum by an alternating current voltage Vmax in accordance with a contrast of |Vmax−V1|. The electrically conductive fine particulate matters removed from the toner are flown onto a portion with a non-image potential Vd by an alternating current voltage Vmix according to the contrast |Vmin−V1| (hereinafter referred to as a “electrically conductive fine particulate matter flying bias”). The electrically conductive fine particulate matters are also supplied to a non-image portion on the photosensitive drum by these actions, whereby conductivity of a charging roller is maintained.
In addition, FIG. 11 schematically shows a situation in which a bias in the direction of flying the electrically conductive fine particulate matters to the non-image portion on the photosensitive drum (a bias in the direction of not flying the toner to the latent image portion) is applied to the developing sleeve and electrically conductive fine particulate matters 41 removed from the toner are being flown to the non-image portion on the photosensitive drum.
Further, when te bias shown in FIG. 11 is applied, the electrically conductive fine particulate matters are flown only when F1>F2, where F1 is a force prompting the electrically conductive fine particulate matters to be removed and flown from the toner and F2 is a force of the electrically conductive fine particulate matters sticking to the toner. Therefore, the following description is limited to the flown electrically conductive fine particulate matters.
The electrically conductive fine particulate matters 41 shown in FIG. 11 are charged to have a positive polarity mainly by the rubbing against the toner. However, since not all the electrically conductive fine particulate matters rub with the toner in the same manner, different electrically conductive fine particulate matters have different triboelectricity.
Electrically conductive fine particulate matters 41a, 41b and 41c shown in FIG. 11 are electrically conductive fine particulate matters having different triboelectricity, respectively.
The electrically conductive fine particulate matters 41 has lower triboelectricity in the following order: the electrically conductive fine particulate matter 41a>the electrically conductive fine particulate matter 41b>the electrically conductive fine particulate matter 41c. 
Here, the toner and the electrically conductive fine particulate matters are flown with a fly prompting force F1=ma that is a product of a mass m and an acceleration a according to a bias applied to the developing sleeve. At this point, F1 can also be represented as F1=qE using a product of an electric field intensity E generated by the applied bias and a charge quantity q that the toner and the electrically conductive fine particulate matters have.
In addition, a distance L that the toner and the electrically conductive fine particulate matters are flown (hereinafter simply referred to as a fly amount L) is represented by the following expression when a time during which the electric field intensity E for flying them by the applied bias is applied is t.L=(½)×at2
When a is found from the above-mentioned two expressions for prompting flying, a=(q/m)×E because ma=qE. Here, (q/m) is so-called triboelectricity.
That is, it is seen that the flying amount L of the toner and the electrically conductive fine particulate matters is proportional to each value of triboelectricity of the toner and the electrically conductive fine particulate matter.
Therefore, as shown in FIG. 11, the flying amount L of the electrically conductive fine particulate matters becomes smaller in the following order: the electrically conductive fine particulate matter 41a>the electrically conductive fine particulate matter 41b>the electrically conductive fine particulate matter 41c. 
That is, when the configuration of FIG. 11 is used, although electrically conductive fine particulate matters corresponding to the electrically conductive fine particulate matter 41a can reach the surface of the photosensitive drum, those corresponding to the electrically conductive fine particulate matter 41b or the electrically conductive fine particulate matter 41c are less likely to reach the surface of the photosensitive drum. As a result, the supply of toner and electrically conductive fine particulate matters to the charging roller 2 is insufficient and charging defaults occur.
Here, as means for increasing the flying force of the electrically conductive fine particulate matters 41, there is a method of increasing the electric field intensity E. The electric field intensity E is represented as E=V/d, where V is an electrically conductive fine particulate matter flying bias, d is an S-D gap. That is, the electric field intensity E can be increased simply by changing the electrically conductive fine particulate matter flying bias or the S-D gap that is an element of the electric field intensity, whereby the flying force of the electrically conductive fine particulate matters 41 can be increased.
Therefore, experiments were conducted by increasing Vpp of an alternating current voltage of the developing bias to make the electrically conductive fine particulate matter flying bias larger or simply making the S-D gap smaller to increase the electric field intensity E. Then, although a Vd maintenability for maintaining a predetermined Vd was improved, a defective image due to a bias leakage of the developing bias onto the photosensitive drum (hereinafter referred to a “leak image”) occurred, in particular, under a low atmospheric pressure in the order of 525 mHg.
Here, a relation between the leak image and the electric field intensity E was examined under the atmospheric pressure of 525 mHg, and it was found that there was a relation as shown in FIG. 13. It is seen from FIG. 13 that both the large electrically conductive fine particulate matter flying bias and the small S-D gap those increased the electric field intensity tend to cause a leak image. That is, when a tolerance of the developing bias, the S-D gap or the like is taken into consideration, it is not very favorable to simply increase the electric field intensity because it makes a margin with respect to the occurrence of a leak image smaller.